The present invention relates to disk drive servo control. More particularly, the present invention relates to improved methods and apparatus for positioning a read/write (R/W) head of a disk drive based on servo burst amplitudes.
Hard disk drives have long been employed for storing data in computer systems. In disk drive servo control, some technique is typically employed to servo the read/write (R/W) head to a particular position on the disk to either write data to a specified region on the disk or to read data from the specified region. Accurate positioning of the head to read or write the data is a challenging task, given the possibility of shock, vibration, temperature and/or manufacturing related variations. To counter the effects of these variables and to ensure accurate positioning of the head, some hard drives employ embedded servo marks and servo bursts for positioning the head in a desired region of the disk for reading or writing.
To facilitate discussion, FIG. 1 depicts a simplified diagram of a hard disk drive 100, typical of hard disk drives known in the art, including a hard disk surface 102. A read/write (R/W) head 104 couples to a suspension arm 106, which is urged by a motor 108 to move R/W head 104 between inner diameter (ID) 110 and outer diameter (OD) 112 of disk surface 102. Motor 108 is shown coupled to a servo circuit 114, which furnishes servo control information to motor 108 to enable motor 108 to position RIW head 104 at a desired location on disk surface 102. Although only one disk surface 102 and one suspension arm 106 are shown for simplicity, it should be understood that there may exist multiple disks, each of which may have both surfaces available for data storage, as well as multiple suspension arms in a typical hard disk drive.
The area between inner diameter (ID) 110 and outer diameter (OD) 112 of disk surface 102 is typically divided into tracks in the form of concentric rings, e.g., tracks 116, 117, 118, and 119. These tracks, which enable data to be located radially, typically go all the way around the disk surface although only a portion of each track is shown in FIG. 1 to simplify the illustration.
Disk surface 102 may also be divided into wedges, or sectors, to enable the data to be located circumferentially about the disk. One such data-storing sector is illustrated in FIG. 1 as sector 126. In between adjacent data-storing sectors, there may be provided a servo wedge, e.g., servo wedge 128 in FIG. 1, for storing positioning data to assist servo circuit 114 in ascertaining the current position of the head. Such positioning data may include, for example, the track number and the sector number. By reading the positioning data, servo circuit 114 may ascertain where on disk surface 102 R/W head 104 is currently positioned.
The above-described technique of positioning the R/W head may permit the head to be placed at the desired sector/track. For some drives, this degree of positioning accuracy is all that is needed. For other drives, however, a greater degree of accuracy is needed. By way of example, in some modem hard drives in which the tracks are reduced in width and packed closely together to increase density, it may be necessary not only to position the head at a particular track but also at a specified position within a track, e.g., at the track center, to minimize noise when reading or writing data. In such drives, embedded servo bursts may be employed to facilitate the required center-track positioning.
FIG. 2 depicts an enlarged view of servo wedge 128 at tracks 116, 117, 118, and 119 of FIG. 1. For simplicity of illustration, the data pertaining to the sectors and tracks have been omitted. Along the tracks within the servo wedge, there are shown a plurality of servo bursts, A, B, C, and D, which are typically written onto surface 102 of the disk during manufacturing. The servo bursts are offset in the track-width direction by, for example, a half track each. Servo bursts A, B, C, and D are in a quadrature arrangement and form a contiguous set, which repeats every other track as shown in FIG. 2. By reading the servo bursts as the R/W head is servoed from track to track, it is possible to ascertain the current position of the R/W head from the servo burst amplitudes.
To further illustrate the foregoing, FIG. 3 plots the amplitudes of the servo burst signals as the R/W head moves across the tracks. For example, consider the situation wherein the head is positioned at track center 204 (of track 116) and moves downward in FIG. 2. The amplitudes of servo bursts A0, B0, C0, and D0 are as follows. At track center line 204, the amplitude of the signal obtained from servo burst B0 will be at its maximum (since the head is located at the center of servo burst B0). However, as the head is moved away from this center of servo burst B0, the amplitude of the signal associated with servo burst B0 decreases from the maximum value. This is shown in FIG. 3 as the amplitude of servo burst B0 decreases as the R/W head moves from line 204 toward line 206 (which delineates the boundary of track 117).
At track center 204, the amplitude of the servo burst D0 will be low since the head is positioned away from servo burst D0. This is shown in FIG. 3 around the vicinity of line 204 (i.e., the track center of track 116 as shown in FIG. 2). However, as the R/W head moves downward in FIG. 2, it moves closer to servo burst D0. Accordingly, the amplitude of servo burst D0 increases as the R/W head moves from line 204 toward line 206 in FIG. 3 (which delineates the boundary of track 117).
At track center 204, the amplitude of the servo burst A0 will be decreasing since the head is moving away from servo burst A0. This is shown in FIG. 3 around the vicinity of line 204 (i.e., the track center of track 116 as shown in FIG. 2). As the R/W head moves downward in FIG. 2, it moves further away from servo burst A0. Accordingly, the amplitude of servo burst A0 decreases as the R/W head moves from line 204 toward line 206 in FIG. 3 (which delineates the boundary of track 117). The amplitude of servo burst A0 reaches its minimum at the vicinity of line 206 and increases again as it approaches servo burst A1 (shown in FIG. 2).
At track center 204, the amplitude of the servo burst C0 will be increasing since the head is moving toward the center of servo burst C0. This is shown in FIG. 3 around the vicinity of line 204 (i.e., the track center of track 116 as shown in FIG. 2). As the R/W moves downward in FIG. 2, it moves closer to the center of servo burst C0. Accordingly, the amplitude of servo burst C0 increases as the R/W head moves from line 204 toward line 206 in FIG. 3 (which delineates the boundary of track 117). The amplitude of servo burst C0 reaches its maximum at the vicinity of line 206 and decreases again as it continues to move away from the center of servo burst C0 (shown in FIG. 2).
To servo to the track center of track 116 (line 204 in FIG. 2), for example, the servo circuit may first servo the R/W to track 116 (by, for example, reading the aforementioned sector and track data in the servo wedge and moving the head accordingly). It then reads the amplitudes of the servo bursts to ascertain the current position of the head. The relationship between individual servo burst measurements allows the servo circuit to calculate the radial distance by which the head needs to move in order to position the head at the center track position. By way of example, the servo circuit may move the head to the point where it calculates the amplitudes of servo bursts A0 and C0 would be equal to each other. With reference to FIG. 2, if the calculation is correct, this position would correspond to line 204, i.e., the center of track 116.
It has been recognized that individual servo burst measurements may at times be susceptible to variations in the environment, e.g., the temperature, the sensitivity of the head, the manufacturing tolerance, the distance from head to disk, and/or other factors. It is also known that the use of signals representative of the difference between amplitudes of complementary servo burst pairs substantially reduces the influence of the environmental factors, thereby reducing the need for calibration and improving positioning accuracy.
FIG. 4 plots the difference between amplitudes of complementary servo burst pairs as the R/W head moves across the tracks. With reference to FIGS. 2 and 3, the complementary servo burst pairs are A/C and B/D. When the head is positioned at line 202 (which delineates the start of track 116), the amplitude of servo burst A will be at maximum and the amplitude of servo burst C will be at minimum. According, the value of signal A-C will be at its maximum, as shown in FIG. 4.
Further, the value of signal A-C will be at zero around the vicinity of line 204. With reference to FIG. 2, it can be seen that the amplitudes associated with servo bursts A and C are substantially equal at this center track line 204. By servoing the head to the position where the value of signal A-C equals zero, the servo circuit is thus able to position the head at the track center. The use of the differential signal reduces the need for calibration since it is recognized that environmental factors tend to affect the measurements of both servo bursts A and C. By subtracting their values from one another, the effect of the environmental factors may be balanced out and/or substantially reduced.
As the drive becomes smaller and/or the storage capacity increases, the adjacent tracks become increasingly narrower and more tightly packed together. At the same time, the dimension of the head shrinks to accommodate the smaller tracks. It has been found with the smaller form factor drives that it is sometimes necessary to servo the head not only to the half track position but also to the quarter track position. With reference to FIG. 2, the quarter track positions within track 116 are depicted by lines 203 and 205, which are 1/4 track away from track boundaries 202 and 206 respectively. The ability to accurately servo the head to the quarter track position is particularly useful in drives which employ magneto-resistive (MR) heads since the read and write heads are separately located (typically fore and aft relative to the suspension arm) and may therefore be offset from one another relative to the track.
It has been found, however, that prior art techniques of calculating track position based on servo burst amplitudes are subject to discontinuity at the point where the calculation switches from one set of servo bursts to the next. To explain the foregoing, consider the manner in which the prior art calculates the Position Error Signal (PES) from the servo burst amplitudes. As the term is employed herein, the Position Error Signal (PES) refers to the offset of the head from either a track edge or a track center. With reference to FIG. 2, for example, the PES refers to the offset of the head from either line 202, 204, or 206 of track 116. The combination of a PES value and the reference line (e.g., either line 202, 204, or 206) permits the servo circuit to ascertain the current position of the head and thus the calculated distance by which the head must move in order to be positioned at the quarter track position.
To further explain the foregoing, consider the situation wherein the head is between track boundary 202 and quarter track line 203, and the values of B-D is currently 0.5 volt (as read from servo bursts B and D). The measured value of B-D of 0.5 volt is shown in FIG. 4 at point 402. With reference to FIG. 3, it is shown that the amplitude of the B servo burst is rising in this region between lines 202 and 204 while the amplitude of servo burst D is falling in this region. In the prior art, the head position is calculated by the following expression, referred to herein as "Equation 1," or "Eq. 1": ##EQU1## wherein n represents an integer. The expression 0.5*n represents the reference line from which the head is offset. As mentioned earlier, this reference line may be either a track boundary or a track center line (as indicated by the value 0.5 in the expression 0.5*n). With reference to FIG. 2, the reference line may be, for example, either line 202, 204, 206, or 208 depending on the value of n.
The expression ((R-F)/(4*K)) represents the PES, i.e., the offset from the reference line, wherein R represents the amplitude of the rising servo burst and F represents the amplitude of the falling servo burst. In the present example in which the head is located between lines 202 and 203, R represents the amplitude of the rising B servo burst while F represents the amplitude of the falling D servo burst.
The value K in Eq. 1 represents the quarter track normalization value which translates the measured value of R-F (measured in volts) into an offset value within a quarter track. By way of example, if the value K is 0.7 volts (as shown in FIG. 4), the ratio (R-F)/K would yield the amount of offset within a quarter track. For example, if the measured value of R-F (e.g., the amplitude of the rising B servo burst minus the amplitude of the falling D servo burst) is 0.5 volts, the ratio of 0.5/0.7 would yield the offset within a quarter track. Dividing the ratio (0.5/0.7) by 4, as is done in Eq. 1, would further normalize the offset value to within one whole track, i.e., yielding the PES. As can be appreciated from those skilled in the art, the combination of the reference half-track position (represented by the expression 0.5*n) and the PES (the expression ((R-F)/(4K)) represents the PES in this case) permits the servo circuitry to ascertain the position of the head from the relationship between the rising and falling servo bursts.
As the head moves from the left side of quarter track line 203 to the right of quarter track line 203, a different set of servo bursts is employed to determine the PES. This is because, as shown in FIG. 4, the differential signal B-D loses its sensitivity and becomes substantially less linear in the region between quarter track line 203 and center track line 204. In the region between quarter track lines 203 and 205, the amplitude of servo burst A is falling and the amplitude of servo burst C is rising, as can be seen in FIG. 3. Accordingly, the differential signal C-A will be employed to represent the value R-F in Eq. 1.
In this disclosure, it is sometimes more convenient and less confusing to employ the reflection of a signal for discussion purposes in the Figures. For example, it is convenient in FIG. 4 to refer to the reflection of curve C-A, which is curve A-C for discussing the servo control issues. Accordingly, points 412 and 414 are shown on respective falling curves B-D and A-C although Eq. 1 shows that they in fact reside on respective rising curves D-B and C-A.
Point 404 is referred to herein as the switch-over point, i.e., the point at which the calculation switches from one differential signal to another differential signal to substantially maintain linearity and sensitivity. As can be seen in FIG. 4, the switch-over point occurs when the magnitude of A-C equals the magnitude of B-D.
In the prior art, the normalization value K is a constant, which may be obtained by ascertaining, for example, the average value of a number or all switch-over points of adjacent tracks on the disk. With reference to FIG. 4, for example, if switch-over point 404 is measured at 0.7 V, switch-over point 406 is measured at 0.65V and switch-over point 408 is measured at 0.75V, the average value of 0.7 V may be employed as the value for K. Such a static K value, however, gives rise to servo discontinuity at the quarter track position.
Consider, for example, the situation wherein the head is at the vicinity of point 406 (i.e., around the quarter track line 205 of FIG. 2). Due to variations in manufacturing and environmental factors, the switch-over point (i.e., the point at which the magnitude of A-C equals the magnitude of B-D) is 0.65V, which is below the static value of 0.7V employed for K in the PES calculation. If the head is at point 410 on curve C-A (since the amplitude of C is rising and the amplitude of A is falling in this vicinity as shown in FIG. 3) and is servoed toward the quarter track position, i.e., to the right of FIG. 4, it will be servoed to the right of the x-axis until the value of 0.65V is read for the differential signal along curve C-A. At this point, the servo circuit believes that it has not yet reached the quarter track position (since the servo circuit employs a normalization factor based on a K value of 0.7V to calculate the PES). If n is arbitrarily assumed to be one for illustrative purposes, this track position is 0.5*1+(0.65/4*0.70) or 0.73, for example.
The servo circuit thus continues to move the head to the right in FIG. 4. As soon as the head moves to the right of line 205, however, the calculation switches from curve C-A to curve B-D (since the amplitude of B is rising and the amplitude of D is falling in this vicinity as shown in FIG. 3). The switch-over occurs since the magnitude of C-A equals the magnitude of B-D as the head moves to the right of line 205.
Since the value of B-D will be near 0.65 immediately after switching, the servo circuit will be under the mistaken impression that the head has overshot the quarter track position and needs to move to the left along the x-axis again. To continue with the earlier example, n is now 2, and this track position is 0.5*2-(0.65/4*0.70) or 0.77, for example. The value of B-D after switching is shown on the B-D curve of FIG. 4 as point 412. This impression is created by the fact that the normalization factor is based on a K value of 0.7V, which makes the value of (R-F)/K less than unity around the vicinity of the quarter track line. Consequently, the servo circuit may go on issuing servo commands which cause the head to hunt to the left and right of quarter track line 205 without ever actually settling on the quarter track line.
As can be appreciated in the prior art, this situation gives rise to servo instability around the quarter track position. Since the prior art technique of ascertaining the PES gives rise to discontinuity around the quarter track position, accurate positioning of the head at this quarter track point proves difficult.
In view of the foregoing, there are desired improved methods and apparatus for reducing the discontinuity around the quarter track position, which leads to servo stability when the head is servoed to the quarter track position.